P-169 Low 5% VS ultra-low 3% O2 concentration on embryo culture: Is there any difference in quality and ploidy?

Study question

Can embryo culture in 3% O2 concentration improve fertilization and blastocyst formation rates, as well as euploidy rate, compared to 5% O2 embryo culture?

Summary answer

Ultra-low oxygen concentration (3%) does not seem to improve fertilization and euploidy rate neither embryo quality and blastocyst formation rate.

What is known already

Low oxygen tension (5%) during embryo culture has found to be beneficial for embryo development, when compared to atmospheric O2 tension (20%). Physiologically, oxygen tension is decreasing from ∼5% in the fallopian tube to ∼2,5% in the uterus and embryos enter the uterus on the post-compaction stage, around Day 4 of their development. Additionally, our previous study demonstrated that euploidy rate was significantly higher in 5% O2 concentration compared to 20%. Thus, the purpose of this study was to evaluate whether culture of embryos to blastocyst stage could benefit from a decrease in O2 concentration, from 5% to 3%.

Study design, size, duration

This retrospective study, included 176 PGT-A cycles, performed at Embryolab fertility clinic from May 2017 to November 2021. Two study groups were formed, in which oxygen tension was the only different parameter of culture conditions; the 5% O2 Group consisted of 114 PGT-A cycles resulting in 832 embryos and the 3% O2 Group with 62 PGT-A cycles including 479 embryos. Euploidy, fertilization and blastocyst formation rates were compared between the two groups.

Participants/materials, setting, methods

In all cycles included in the study, injected oocytes were incubated to either 3% O2 or 5% O2. The CO2 concentration was 6% in both groups. Embryos were cultured to blastocyst stage. In good quality available blastocysts (expansion: >3, ICM: A/B, TE: A/B according to Gardner Grading System), trophectoderm biopsy was performed on Days 5/6 and embryos were analyzed for chromosomal abnormalities. All embryos were vitrified after biopsy. Statistical analysis was performed using Mann-Whitney test.

Main results and the role of chance

Mean female age was similar between 5% and 3% group (37.76±4.7y vs 38.35±4.19y, p = 0.2263).

Fertilization rates were similar between two groups (81.04% in 5% VS 81.77 in 3%, p = 0.9718). In 5% O2 Group, cleavage rate (98.86%) was found to be significantly higher, compared to 3% O2 Group (96.86%) (p = 0.003). Blastocyst formation rate (expressed per fertilized oocyte) was found to be significantly higher in the 5% O2 Group (76.56 in 5% VS 67.22 in 3%) (p = 0.029), as well as biopsied blastocyst rate (91.9% in 5% VS 82.29% in 3%) (p = 0.0056).

Euploidy rates were found to be similar between the two groups, 26.7 in 5% Group and 26.76 in 3% Group (p = 0.9272). However, more blastocysts were available for biopsy on Days 5/6 in the 5% Group, as shown above.

In the 5% Group, 101 embryotransfers have been performed, 54 with positive βHCG and 42 cycles had no euploid embryos after PGT-A. In the 3% Group, 28 embryotransfers have been performed, 18 with positive βHCG, 7 cycles are pending and 24 had no euploid embryos. This is an ongoing study and clinical outcomes remain to be reported and analyzed after the completion of data selection.

Limitations, reasons for caution

This is a retrospective study in which cycles from the two groups were not performed simultaneously but at different time periods. In addition, all PGT-A cycles were included in the study, regardless the type of infertility (e.g. both male and non-male factor infertility).

Wider implications of the findings

Based on these results, it is not recommended to decrease oxygen tension below 5% during culture, as this does not improve fertilization and euploidy rates. Nevertheless, embryo quality and blastocyst formation was found to be higher in 5% Group and this findings should be confirmed in a larger sample size.

Trial registration number

not applicable

P–224 Multinucleated and non-multinucleated embryos in PGT-A cycles: Is there any difference?

Study question

Is multinucleation during cleavage stage correlated with the ploidy status of embryos and how does it affect the clinical outcome?

Summary answer

The presence of multinucleated embryos does not affect clinical outcome, although the risk of aneuploidy is higher in multinucleated embryos.

What is known already

Multinucleated blastomeres (ΜΝ) of cleavage stage embryos has been reported widely in scientific literature. Multinucleation has been associated with diminished embryo developmental competency and clinical outcomes such as lower implantation. Although this is an intriguing subject of research, it is not clear yet whether multinucleation is related to aneuploidy. Morphological irregularities such as multinucleation in blastomeres became a constant finding only after the perpetually evolving technology of time-lapse culture of embryos which in combination with PGT-A analysis, creates new research paths which aim to develop a new tool for selection or deselection of embryos for transfer.

Study design, size, duration

This retrospective study, included 97 PGT-A cycles, performed at Embryolab fertility clinic from May 2017 to December 2020, all cultured in time-lapse incubator (EmbryoScope). Two study groups were formed; the MN Group consisted of PGT-A cycles with at least one multinucleated embryo (n = 56) and the Control Group in which all PGT-A cycles had no multinucleated embryos (n = 38). Euploidy rate, type of chromosomal abnormality, cumulative pregnancy and live birth rates were compared between the two groups.

Participants/materials, setting, methods

Embryos were annotated for the existence of multinucleated blastomeres on Day 2 of their development. Biopsy was performed on Days 5/6 and embryos were genetically tested. One or two euploid embryos were transferred. Euploidy rate and clinical outcomes between the two groups were compared. Within the MN group, euploidy rate between multinucleated and non-multinucleated embryos was compared. For abnormal embryos, association of multinucleation with the type of abnormality was tested. SAS statistical analysis was performed.

Main results and the role of chance

Mean female age was 35.93 years in the MN group and 38.39 years in the control group. Blastocyst formation rate (expressed per fertilised oocytes) was similar between MN and Control group (74% vs 76%, p = 0.6303). In the MN group, 56 cases resulted in 44 embryo transfers while in the control group 38 cases resulted in 23 embryo transfers. Pregnancy rates (59.09% vs 65.21%, p = 0.6255) and clinical pregnancy rates (45.45% vs 39.13%, p = 0.4245) were not significantly different between MN and Control group. Initially, cumulative live birth rate was found to be significantly higher in the MN group compared to the Control group (62.96% vs 33.34%, p = 0.0417). However, when logistic and poisson regression was applied, it became obvious that this difference was not affected by multinucleation but from other factors such as female age. When comparing multinucleated and non-multinucleated embryos within the MN group, it was found that the mean number of euploid embryos was significantly higher in the non-multinucleated subgroup of embryos (p = 0.0021). No correlation was found between multinucleation and the type of chromosomal abnormality.

Limitations, reasons for caution

The sample size is the main limitation of the present study. More research with bigger sample size is needed in order to confirm the finding of the present study.

Wider implications of the findings: The present study suggests that multinucleated blastomeres during embryo development is not an indication for diminished blastocyst formation and does not affect the clinical outcomes. However, within a sibling embryo population, non-multinucleated embryos tend to be euploid and this finding can be used to advance embryo selection efficiency.

Trial registration number

Not applicable